[docs] Link from QtConcurent::run() to QThreadPool::start(Callable&&)

The latter is a better choice if you don't need the result QFuture.

Describe the QThreadPool::start() overload in prose, since the
signature changed in 6.6 from std::function<void()> to Callable
auto&&. The chosen wording is compatible with both.

Fixes: QTBUG-111875
Change-Id: I9f67c2f7e4b221602bf1c35b72e5d38898a4f0c9
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 47a0282707d6a0d12f6f584c81a982632d09cfb7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/concurrent/qtconcurrentrun.cpp

@@ -80,6 +80,10 @@
 
     \snippet code/src_concurrent_qtconcurrentrun.cpp 2
 
+    If you don't need the result (for example, because the function returns
+    \c{void}), using the QThreadPool::start() overload taking a function object
+    is more efficient.
+
     As documented above, passing arguments is done like this:
 
     \snippet code/src_concurrent_qtconcurrentrun.cpp 3
@@ -251,7 +255,7 @@
     running task, fetching multiple results from the called \a function or
     monitoring progress reported by the \a function.
 
-    \sa {Concurrent Run (basic mode)}, {Concurrent Run With Promise}
+    \sa {Concurrent Run (basic mode)}, {Concurrent Run With Promise}, QThreadPool::start()
 //! [run-description]
 */